An aerofoil structure is a body, such as an aircraft wing or turbine blade, designed to provide a desired reaction force when in motion relative to a surrounding working medium. Some aerofoil structures, such as aircraft wings and wind turbine blades, generate lift perpendicular to the surrounding airflow. Others, such as racing car wings, generate downforce. Other examples of aerofoil structures include propeller blades, helicopter rotor blades, sailboat centreboards, and aircraft vertical and horizontal tail planes and control surfaces.
Aircraft wings, and many other aerofoil structures, typically comprise a thin outer skin supported by a reinforcing inner framework and are known as a semi-monocoque. The upper and lower skins, or covers, of a wing support the aerodynamic pressure distribution. These aerodynamic forces are transmitted to the reinforcing framework, which typically includes longitudinal stiffening spars and stringers, and transverse ribs. The spars include a spar web between upper and lower spar caps or flanges. The ribs resist the aerodynamic pressure loads to maintain the aerofoil cross-section shape, and also distribute concentrated loads such as from the undercarriage. The stringers are attached to the skin and ribs to divide the skin into small panels. The skin and spar webs act together to resist the shear and torsional loads; and the skin, spar caps and stringers act together to resist the axial and bending loads.
This typical aircraft wing construction provides advantages in terms of manufacturability and maintenance but has shortcomings as the main structural spar and rib elements do not always follow the lines of maximum load leading to structural inefficiency. This is particularly the case for swept aircraft wings commonly used for commercial airliners which operate at transonic speeds. Torque on swept wings leads to sub-optimal loading of ribs that run from the leading edge to the trailing edge of the wing, generally perpendicular to the main spar. With recent shifts away from metallic aircraft wing design towards composite or plastic structures, some benefits of these new materials are currently not being exploited by following traditional wing designs.